The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to the plurality of torque transmitting devices within the transmission. The pressurized hydraulic fluid delivered to the torque transmitting devices is used to engage or disengage the devices in order to obtain different gear ratios.
In order to increase the fuel economy of motor vehicles, stopping the engine during certain circumstances may be desirable, such as when stopped at a red light or idling, which may improve fuel economy. The engine may also be stopped while the vehicle is in motion for cruising, or if there is another source of torque, such as a motor in the rear axle. However, in certain circumstances, the stopping of the engine is undesirable.
In addition, after the engine has been shut down and has remained off for an extended period of time, the fluid generally tends to drain down from the passages into a transmission sump under the force of gravity. Upon engine restart, the transmission may take an appreciable amount of time to establish pressure before full transmission operation may resume. Quick system recovery, however, is desirable. As such, maintaining pressure in a clutch could aid in quick system recovery, even when line pressure and/or clutch feed pressure is zero or near zero. In other situations, however, clutch pressure should be eliminated, such as when putting a vehicle in reverse.
Accordingly, there is a need for a method of controlling engine stop-starts while maintaining timely shifting and recovery.